Piles utilize both end bearing and skin friction on the outer surface of the pile to obtain adequate load capacity. The performance of piles has historically been better than for other shallower foundation systems. However, under certain soil conditions, piles can have problems. Specifically, piles are sometimes used in areas having thick layers of underconsolidated soil sandwiched between stiff soil at the surface and stiff or dense soil at a significant depth. As this middle layer of underconsolidated soil continues to compress or consolidate over time, the subsequent downward movement of the upper stiff layer of soil creates negative skin friction on the upper portion of the pile, thereby increasing the downward load. This increased downward load has been known to cause failure of piles.
Another soil condition that can cause problems with piles occurs when soil shrinkage from extremely dry weather causes a gap to develop between the soil and the pile surface. This shrinkage gap causes a loss of skin friction in the upper portion of the pile, thereby reducing the capacity of the pile, sometimes to the point of failure. Since there is loss of skin friction, the original calculations made to determine the amount of support for the structure can grossly underrepresent the ultimate capacity provided. Since there is a loss of skin friction in the upper portion of the pile, the only support for the structure will come from skin friction and bearing in the lower portion of the pile.
Still another soil condition that can affect these piles is soil heave or swelling. In particularly cold weather climates, the soil freezes during cold winter months. Whenever the soil freezes, the soil within the frost zone can expand due to freezing. In areas with extremely expansive soils, the soil can swell during very wet periods. Both freezing and wetting of soils can cause a heaving action on the pile which can permanently damage the pile. As such, if skin friction exists between the pile and the earth in the upper portion, then damaging uplifts of the pile can occur.
In the past, various patents have issued relating to the skin friction affecting such piles.
U.S. Pat. No. 4,070,867, issued on Jan. 31, 1978 to F. G. Cassidy, describes a building pile structure and system that utilizes a skin friction pile having a casing or sleeve of somewhat larger diameter than the outside diameter of the pile. This casing or sleeve is driven over the pile either simultaneously with the driving of the pile or driven somewhat in advance of the pile so as to isolate the pile from certain areas of the surrounding soil for a portion of the total depth into which the pile is driven.
U.S. Pat. No. 4,585,681, issued on Apr. 29, 1986 to Kidera et al., describes a frost damage-proof pile for installment in a frigid region where the pile is subjected to a freezing and frost heaving force, such as which occurs with permanently or seasonally frozen soil terrain. A tubular sheath member is fitted over the pile surface and has a length longer than the thickness of an active or seasonally frozen soil layer of the terrain in which the pile is installed. At least a portion of the length of the pile is formed as an extensible section, and at least the lower end of the sheath member is secured to the pile at or below a position corresponding to the bottom region of the active or seasonally frozen soil layer. A fluid material is filled into the space defined between the pile and the sheath member. The frost heaving force caused to exist upon freezing of the active or seasonally frozen soil layer as well as negative friction caused to exist in summer are inhibited from affecting the pile due to sliding of the sheath member relative to the pile.
U.S. Pat. No. 4,818,148, issued on Apr. 4, 1989 to Takeda et al., describes a frost damage-proofed pile in which a covering is applied onto the outer surface of the pile. This covering includes a steel pipe which surrounds a predetermined length of the pile so as to reduce a frost heaving force or negative friction acting on the pile in a frigid area. The covering is closely adhered by an adhesion layer to the pile over a given length thereof. The covering member includes a smooth-surfaced plastic covering or elastic covering. A rugged surface covering may be provided below the smooth surfaced covering.
It is an object of the present invention to provide a method and apparatus for avoiding the problems associated with adverse skin friction in the upper portion of the pile shaft.
It is still another object of the present invention to provide a method and apparatus which reduces or eliminates the effects of negative skin friction.
It is another object of the present invention to provide a method and apparatus which serves to reduce or eliminate the effects of shrinkage induced loss of contact with the soil.
It is still another object of the present invention to provide a method and apparatus which reduces or eliminates the effect of soil heave due to swelling clays or freezing of soil in a frost zone.
It is still another object of the present invention to provide a method and apparatus which is easy to install, relatively inexpensive and easy to manufacture.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.